1. Field of the Invention
The present invention relates to a semiconductor laser diode (hereafter denoted as LD), in particular, the invention relates to a tunable LD.
2. Related Prior Art
A Japanese Patent Application published as JP-H07-273400A has disclosed a tunable LD having a sampled grating (hereafter denoted as SG) and an optical waveguide in which a plurality of gain regions and tuning regions are alternately arranged along the optical axis of the LD. Such a tunable LD may shift peak wavelengths appeared in the discrete spectrum caused by the SG by modifying the refractive index in the tuning region.
The tunable LD in the prior patent inevitably includes many interfaces between the gain region and the tuning region. Such an interface is often called as the butt-joint derived from the manufacturing process thereof. The butt-joint generally has an optically disarranged or discontinued interface to cause unintentional internal reflection thereat, which degrades the performance of the tunable LD. Accordingly, the number of the butt-joint should be reduced as possible.